1. Field of the Invention
The present invention relates to a press type method and apparatus for reducing the slab width in which the width of a slab as a rolling stock is reduced before rolling, and more particularly, to a press type method of reducing the slab width improved in efficiency and formability.
2. Description of the Prior Art
These days, a flat stock for manufacturing a hot rolled strip, i.e., a slab is often obtained by means of a continuous casting machine. In this case, since it is difficult to directly vary the slab width by means of a continuous casting machine on the grounds of formability, there is a need for an extra process for regulating the slab obtained by a continuous casting machine to a slab width required according to rolling.
The dimensions of a stock or slab employed in hot rolling, for example, are usually required to fall between wide ranges: the slab thickness ranges from 30 to 300 mm; and the slab width from 900 to 2000 mm.
The slab thickness can be relatively easily regulated to a predetermined dimension required in rolling by employing such an equipment as a thickness-reducing rolling mill.
By a conventional slab width regulating means employing a vertical rolling mill, however, since the rolling roll diameter is 1200 to 2000 mm, it is difficult to apply pressing forces to the central portion of a slab. Consequently, if it is intended to regulate the slab width by a large margin, a projecting edge portion called "dog bone" may be formed at each of ends of the slab in its width direction, resulting in such an irregular shape that only the edges of the slab in its width direction are large in thickness. Therefore, in general, it is only possible to effectively perform a reduction in slab width within about 100 mm.
Because of this, a press type method of reducing the slab width has recently been devised that while a slab is fed longitudinally, compressive forces are applied to both edges thereof in the slab width direction by means of press tools each having a flat press surface thereby to reduce the width of the slab.
More specifically, as shown in the specification of Japanese Patent Laid-Open No. 10363/1980, for example, press tools adapted as if they were rolling rolls each having an infinite radius are employed to apply compressive forces over a wide range simultaneously, thereby to prevent the production of the above-mentioned "dog bones".
The conventional method, however, has problems of efficiency and formability, since the method employs a parallel press in which the press surfaces of press tools are parallel to each other. More specifically, in the case of such a parallel press, it is not possible to extremely increase the width of the slab side surface that can be pressed in a single operation owing to limitations of the required press forces. On the other hand, if the width of each press surface of the parallel press is reduced to decrease the required press force, there is a need for a remarkably large number of pressing operations. In other words, in case of employing such a parallel press, every time the slab is pressed the feed thereof is suspended. After pressing, the press tools are separated from each other to release the slab, and under this state, the slab is fed to the amount of corresponding to the press surface width. Therefore, even if the press surface width is set within a range where a proper press force can be applied, time is required for suspending and positioning the slab with the intermittent feed of the slab. In particular, when the slab is suspended, the feed speed must be gradually decreased, as a result the working efficiency is remarkably reduced. Moreover, since the feed of the slab cannot be started unless the press tools are opened more than the former width of the slab after pressing, a larger clearance is required between the press tools as the slab width regulation is larger in amount, which also consumes time.
In addition, since much time is required for the operation for reducing the slab width as described above, there are cases where the slab cools down during the operation to a temperature lower than a predetermined temperature required for processing. Moreover, in the above slab width reducing method employing the parallel press, there is a trouble which because the continuity of the boundary area between the surface portions pressed in successive operations is poor, an edge crack is caused in the thickness-reducing rolling operation carried out in the subsequent step.